Application sending JSON on demand, not on creation (Ext-JS 4)

When I start my application, my computer loads it 15 seconds. It loads a whole bunch of grids and things that I want to load dynamically.What do I have to do if I want for those grids to be loaded "on demand"? Do I have to load controllers dynamically or just the grids? And how?Thank you.

From my experience dynamic loading is always slower then loading everything at once in one file. Split files are nice for developing, but not for production.
Of course, it requires some time to create classes, but much less then Ext itself and mostly equal to network latency of one-two requests. Sencha Tool can automate build process, if properly configured.